Crystalline polyesters have been recently noticed as a resin binder suitable for improving a low-temperature fusing property of toners because they have a high compatibility with non-crystalline polyesters and are readily dispersed therein, and exhibit a clear melting point owing to their crystalline moieties, unlike other crystalline resins such as polyethylene.
JP-A-2009-139588 (“JP-A” means unexamined published Japanese patent publication) discloses a toner for developing an electrostatic image which contains core/shell type toner particles each composed of a core layer containing at least a crystalline resin, a first resin binder, a releasing agent and a colorant and a shell layer containing a second resin binder, in which the crystalline resin has an endothermic peak temperature of from 25 to 50° C., a total content of the crystalline resin in the toner particles is from 3 to 15% by weight, and the toner particles have an acid value of 20 mg KOH/g or less, for the purpose of providing a toner for developing an electrostatic image which exhibits an excellent low-temperature fusing property and a good charging property even under high-humidity environmental conditions.
JP-A-2009-075342 discloses a toner for developing an electrostatic image which contains at least a crystalline polyester resin and a colorant, and has a dielectric loss factor ∈″ of 0.1 or less as measured under the conditions of 0.1 Hz and 500 V at 30° C. and 90% RH, for the purpose of providing a toner for developing an electrostatic image which is capable of maintaining a low-temperature fusing property even under high-temperature and high-humidity conditions, forming a high-density image and suppressing occurrence of fogging.
The conventional core/shell toner particles containing a crystalline polyester obtained by polycondensing an alcohol component containing an aliphatic diol having 2 to 12 carbon atoms and a carboxylic acid component containing an aliphatic dicarboxylic acid compound having 8 to 12 carbon atoms in an amount of from 70 to 100 mol % are excellent in low-temperature fusing property, but have problems such as contamination of a carrier used therewith and a low charging rate.
JP-A-2009-139588 and JP-A-2009-075342 both disclose the core/shell toner particles containing the crystalline polyester, but fail to specify the above problems and provide a means for solving the problems.